The transmission mechanism of an automatic power transmission for an automotive vehicle has incorporated therein a plurality of frictional units which are to be selectively made operative and inoperative to make shifts between various forward and reverse drive gear ratios, as is well known in the art. In an automatic power transmission having three forward drive and one reverse drive gear ratios, such frictional units typically include a high-and-reverse clutch, a forward drive clutch, a brake band, and a low-and-reverse brake. Of these transmission clutches and brakes, the forward drive clutch in particular is maintained coupled throughout the conditions in which the gear ratios in the automatic and manual forward drive ranges are to be selected and established in the transmission mechanism. If all the other frictional units than the forward drive clutch are held inoperative, the transmission mechanism is conditioned to produce the first gear ratio in the automatic forward drive range in cooperation with a one-way clutch also incorporated into the transmission mechanism. If the brake band is put into operation additionally to the forward drive clutch, then the second gear in the automatic or manual forward drive range is selected depending upon the position to which the manually operated transmission gear shift lever has been moved. If the brake band is thereafter released and, in turn, the high-and-reverse clutch is actuated to couple, a shift is made from the second gear ratio to the third gear ratio in the automatic forward drive range. If, on the other hand, the high-and-reverse clutch and the low-and-reverse brake are in operation concurrently with the forward drive clutch held inoperative, then the reverse drive gear ratio is selected in the transmission mechanism.
The clutches and brakes thus predominant over the shifting between the various gear ratio conditions of the transmission mechanism are hydraulically operated by means of a hydraulic control system which is basically operated by manipulating the above mentioned transmission gear shift lever which is usually installed on the floor, the steering column or otherwise of an automotive vehicle. More specifically, each of the clutches and brakes provided in the transmission mechanism is operated by a control fluid pressure, or line pressure, produced by a pressure regulator valve incorporated into the control system. In the hydraulic control system of a known automatic power transmission, the line pressure thus produced by the pressure regulator is developed on the basis of another control pressure, or throttle pressure, which is continuously variable with the opening degree of the throttle valve provided in the carburetor of an internal combustion engine with which the power transmission is to operate in an automotive vehicle. The clutches and brakes for controlling the shifts between gear ratios are, for this reason, initiated into operation and thereafter held operative with forces which vary with the opening degree of the carburetor throttle valve. On the other hand, the line pressure supplied from the pressure regulator is distributed to the transmission clutches and brakes selectively by and through a suitable number of gear shift valves which are responsive to both the throttle pressure and a third control fluid pressure, or governor pressure, which is continuously variable with the road speed of the vehicle in operation. The transitive points, or shift points, at which shifts are to be automatically made between the gear ratios available are thus determined by the relationship between the vehicle speed and the opening degree of the carburetor throttle valve.
When a shift is to be made from the second gear ratio to the third gear ratio in the automatic forward drive range by means of a prior-art hydraulic control system thus arranged and if, in this instance, the engine is being operated under relatively low loads, the shift points to be determined by the gear shift valves in the hydraulic control system are therefore subject to variation in the vehicle speed although shifting between gear ratios need not be effected in relation to the vehicle speed which is normally maintained at relatively low levels under low load operating conditions of the engine.
The high-and-reverse clutch which is used to select the third gear in the automatic forward drive range or the reverse drive gear position in the transmission mechanism is usually designed to provide a potential torque transmitting capacity which is particularly adequate for the reverse drive gear condition and which is accordingly rather excessive for the forward drive gear condition. During shifting from the second gear ratio to the third gear ratio in the automatic forward drive range, such a high-and-reverse clutch is driven to couple with a force which is exerted by the above mentioned line pressure variable with the opening degree of the carburetor throttle valve. As is well known, however, the opening degree of the throttle valve of an engine carburetor does not accurately reflect the energy output of the engine and accordingly the amounts of load to be applied to the frictional units of the transmission mechanism. The force exerted on the high-and-reverse clutch by the line pressure variable with the opening degree of the carburetor throttle valve is excessive for the load to be imparted to the clutch during shifting from the second gear ratio to the third gear ratio in the automatic forward drive range and, for this reason, it has been unavoidable that shocks are created in the transmission mechanism during an upshift between these gear ratios. In order to solve this problem, it is advantageous to have the high-and-reverse clutch operated by a fluid pressure which is continuously and accurately variable with the energy output of the engine or, in other words, in relation to both the output speed of the engine and the torque output to be delivered from the engine. The forward clutch to be maintained in the coupled condition thereof throughout the forward drive ranges of the transmission system is free from the above described tendency encountered by the high-and-reverse clutch because the forward drive clutch does not lend itself to shifting between the gear ratios in the forward drive ranges and is, accordingly, not responsible for the production of shocks in the transmission mechanism under forward drive gear conditions.
In respect of the shift point from the second gear ratio to the third gear ratio in the automatic forward drive range, furthermore, it will be advantageous to effect such a shift in response to a predetermined vehicle speed without respect to the opening degree of the carburetor throttle valve under low load operating condition of the engine and to vary the shift point between these gear ratios in relation to the vehicle speed and the opening degree of the carburetor throttle valve when the throttle valve opening degree is within a certain range larger than a predetermined limit value.
The present invention contemplates, inter alia, realization of the above described characteristics in controlling gear shifts in the transmission mechanism of an automatic power transmission for an automotive vehicle.